The present invention is in the field of integrated circuits. More particularly, the present invention provides a method and apparatus for accurately controlling the thickness of a semiconductor wafer during a backside grinding process.
A semiconductor wafer generally includes a first or xe2x80x9cfrontxe2x80x9d side having integrated circuits formed thereon, and a backside comprising a thickness of a bulk semiconductor material (e.g., silicon). Prior to the dicing and packaging of the individual integrated circuit chips formed on the front side of the wafer, the wafer is typically ground on the backside to remove unwanted bulk semiconductor material. The backside grinding process reduces the thickness of the integrated circuit chips, allows smaller packaging, provides better stress performance in laminated packages, and provides other known benefits. Control methods for the backside grinding process typically rely on the mechanical precision of the grinding tool to control the accuracy of the final thickness of the wafer. Although wafers are currently typically ground to a final thickness of about 300 micrometers (microns), the thickness requirement is expected to drop to the 200-250 micron range in the near future, with longer term requirements of 100-150 microns or less. Such thickness requirements will be difficult or impossible to achieve without an accurate feedback mechanism to determine the material thickness during the backside grinding process.
During the backside grinding process, the front side of the wafer is often mounted to a protective tape and placed front side down on a chuck. The backside of the wafer is ground away, stopping at a predetermined depth. The thickness of the protective tape must be taken into account to calculate the final thickness of the wafer. Unfortunately, since the actual tape thickness may vary, the measured thickness of the wafer may also vary. Thus, since the tape thickness affects the final wafer thickness, the tape thickness must be held to very tight tolerances, which results in a very expensive commodity.
Another problem with current methods of backside grinding is determining if the wafer is being formed into a xe2x80x9cwedge.xe2x80x9d Wedging of the wafer can occur if the wafer""s surface is not accurately perpendicular to the axis of rotation of the chuck. Wedging can thin one part of the wafer too much, while leaving another part of the wafer too thick. In extreme cases, wedging can actually destroy part of the wafer.
Without a method and apparatus for more accurately reducing wafer thickness without damage to the wafer, the future requirements for wafer thickness will not be met.
The present invention uses optical measurement of the wafer thickness during a backside grinding process to determine the endpoint of the grinding process. This greatly increases the accuracy of the final thickness of the wafer, thereby allowing, inter alia, thinner integrated circuit chips to be produced. Preferably, for a silicon wafer, an infrared interferometric wafer thickness measurement is used.
Generally, the present invention provides a method for thinning a semiconductor wafer, comprising the steps of:
removing a portion of a backside of the semiconductor wafer;
optically measuring a thickness of the semiconductor wafer; and
stopping the removing step when the measured thickness of the semiconductor wafer reaches a predetermined thickness.
The present invention also provides additional preferred methods for controlling a thickness of a semiconductor wafer during a backside grinding process, the methods entailing measuring light transmitted through or reflected by a semiconductor wafer as a function of angle of incidence or of wavelength. This information is then used, through the use of curve fitting techniques or formulas, to determine the thickness of the semiconductor wafer. Furthermore, the present invention may be used to determine if wedging of the semiconductor occurs and, if wedging does occur, to provide leveling information to the thinning apparatus.
The present invention also provides an apparatus for thinning a semiconductor wafer, comprising a system for removing a portion of a backside of the semiconductor wafer, an optical system for measuring a thickness of the semiconductor wafer, and a control system for halting the removing step when the measured thickness of the semiconductor wafer reaches a predetermined thickness.